clear that an eye thus placed at a distance of ten inches from _emf_ (which is about the average distance of distinct vision) would not only receive much more light from the image _emf_, than it would from the object EMF, but see the image much larger than the object. It is in this way that a simple object-glass forms a telescope, a circumstance we shall presently have to notice more at length. But we want to gain the full benefit of the light which has been gathered up for us by our object-glass. We therefore interpose a small convex glass _ab_ (called an eye-glass) between the image and the eye, at such a distance from the image that the divergent pencil of rays is converted into a pencil of parallel or nearly parallel rays. Call this an emergent pencil. Then all the emergent pencils now converge to a point on the axial line _m_M (produced beyond _m_), and an eye suitably placed can take in all of them at once. Thus the whole, or a large part, of the image is seen at once. But the image is seen inverted as shown. This is the Telescope, as it was first discovered, and such an arrangement would now be called a _simple astronomical Telescope_. Let us clearly understand what each part of the astronomical telescope does for us:-- The object-glass AB gives us an illuminated image, the amount of illumination depending on the size of the object-glass. The eye-glass enables us to examine the image microscopically. We may apply eye-glasses of different focal length. It is clear that the shorter the focal length of _ab_, the nearer must _ab_ be placed to the image, and the smaller will the emergent pencils be, but the greater the magnifying power of the eye-glass. If the emergent pencils are severally larger than the pupil of the eye, light is wasted at the expense of magnifying power. Therefore the eye-glass should never be of greater focal length than that which makes the emergent pencils about equal in diameter to the pupil of the eye. On the other hand, the eye-glass must not be of such small focal length that the image appears indistinct and contorted, or dull for want of light. [Illustration: _Fig. 2._] Let us compare with the arrangement exhibited in fig. 1 that adopted by Galileo. Surprise is sometimes expressed that this instrument, which in the hands of the great Florentine astronomer effected so much, should now be known as the _non-astronomical Telescope_. I think this will be readily understood when we compare the two arrangements.